Various processes are known for converting woody and nonwoody lignocellulosic substrates into fibrous products suitable for ingestion by animals and humans.
Cattle, sheep and other ruminants are able to digest and grow on many kinds of cellulosic plant materials that provide little or no nourishment to humans and other monogastrics. Even the ruminants, however, have limited ability to efficiently digest lignocellulosic materials such as the leaves and stalks of grain-bearing grasses and the husks and hulls of the grain. This low conversion efficiency has been attributed to the close association of lignin with the cellulosic and hemicellulosic fibers in these materials. This lignin makes these cellulosics largely unavailable for digestion by the digestive juices and the microbes that inhabit ruminant stomachs. (See Jelks, U.S. Pat. No. 3,939,286 and Gould, U.S. Pat. No. 4,649,113).
Human inability to digest and assimilate cellulose and hemicellulose makes the substrates attractive as potential sources of dietary fiber. But, widespread use for this purpose has been hampered by the lignin that envelops the cellulosic fibers, by the highly crystalline character of the fibers and by the presence of components such as fatty substances (fats and oils) and ash-forming substances (including silicaceous materials). The crystalline character imparts undesirable physical properties to foodstuffs and the fatty and ash-forming substances, especially when used in relatively high proportions, adversely effect the aroma, taste, texture and mouthfeel of food products.
One lignocellulosic material used as a dietary fiber is bran, the unbleached coarse outside covering of the seeds or kernels of cereal grains. Bran is used as fiber or roughage in some breakfast foods, breads and muffins. But, most of the bran is used in animal food, primarily because its high non-cellulosic content adds undesirable properties to many kinds of baked goods, particularly to white bread.
Low calorie flour substitutes made by grinding hulls of oats and other cereal grains (see Tsantir et al., U.S. Pat. No. 3,767,423) contain relatively large proportions of non-cellulosic components such as ash-forming substances. At desirably high flour replacement levels, food products in which they are used have a gritty aftertaste. For this reason, commercial interest has shifted largely to purified cellulose as a dietary fiber for human consumption.
Two forms of purified cellulose, both derived from wood products, are currently available. They are crystalline alpha cellulose, sold under the trade name "Solka-Floc", and microcrystalline cellulose, derived from alpha cellulose, sold under the trade name "Avicel". These products, however, are not entirely satisfactory as flour substitutes (See Glicksman et al., U.S. Pat. No. 3,676,150; Satin, U.S. Pat. No. 4,237,170; Tsantir et al., U.S. Pat. No. 3,767,423; and Torres, U.S. Pat. No. 4,219,580). The taste and texture of baked goods is adversely effected at flour replacement levels greater than about 20 percent.
Gould, U.S. Pat. No. 4,649,113 (1987), discloses a process (Gould Process) for converting nonwoody lignocellulosic agricultural residues (substrate) such as wheat straw into cellulosic fiber products digestible by ruminants and microbes. Gould et al., European Patent Application No. 228951 (1987), discloses that the delignified fiber products of U.S. Pat. No. 4,649,113 are also suitable as noncaloric fiber additives to compositions intended for consumption by humans.
The Gould Process involves slurrying the substrate in aqueous hydrogen peroxide (H.sub.2 O.sub.2) and alkali (NaOH) at a pH of 11.2 to 11.8 and a temperature of 5.degree. to at least 60.degree. C. The substrate is sufficiently delignified exposing virtually all the cellulosic carbohydrates. During the alkaline peroxide treatment, the pH of the reaction medium drifts upward and is controlled by the addition of acid. The H.sub.2 O.sub.2 assists in the delignification of the substrate by oxidizing and degrading lignin to low molecular weight water-soluble compounds, principally carboxylic acids.
Gould et al. teaches that the products can serve as wheat flour substitutes at high (30% or more) replacement levels.
Although attractive as a means of converting substrates to food formulations for ruminants and humans, the Gould Process is not entirely satisfactory for commercial use. It requires rather high concentrations of both H.sub.2 O.sub.2 and NaOH based on the substrate and suffers high losses of H.sub.2 O.sub.2 through nonfunctional (nonproductive) decomposition to oxygen gas (2 H.sub.2 O.sub.2 .fwdarw.2 H.sub.2 O+O.sub.2). Also, we have found that the process when used to treat difficult substrates such as oat hulls results in a rapid decrease in the concentration of the H.sub.2 O.sub.2, accompanied by excessive initial foaming of the reaction mixture, and the production of products that have undesirable quality (brightness, taste and aroma) for human consumption.
Decomposition of H.sub.2 O.sub.2 in a highly alkaline heterogeneous reaction medium, such as when a particulate substrate is present, is not too surprising for a couple of reasons. First, H.sub.2 O.sub.2 is known to be unstable in alkali, particularly at high pH. Second, heterogeneous H.sub.2 O.sub.2 decomposition into H.sub.2 O and O.sub.2 (catalyzed by solid surfaces) is generally far faster than homogeneous decomposition (catalyzed by a variety of soluble, mostly cationic substrates), with the rate increasing in proportion to the surface area of the solids (see Schumb et al., Hydrogen Peroxide, ACS Monograph Series, New York, Rheinhold (1955) pp 521-522).
In a copending application to Jayawant (CH-1459) assigned to E. I. du Pont de Nemours & Company, an improvement over the process of U.S. Pat. No. 4,649,113 is taught for converting nonwoody substrates, particularly nonwoody lignocellulosic agricultural residues, into cellulosic fiber products useful as a source of carbohydrates digestible by ruminants and as a source of low calorie dietary fiber ingestible by humans. The process broadly comprises treating lignocellulosic substrates in an aqueous solution of strong alkali (Alkaline, Peroxide-Free Stage) for a period of time prior to the addition of peroxide (Alkaline-Peroxide Stage). Both the copending application and U.S. Pat. No. 4,649,113 are incorporated herein by reference.